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LM35: Dilemma-Design Application Note SNIS159H -Figure 14 versus figure 12

Seamus Sheehy
Prodigy 20 points
Part Number: LM35

I recently had an issue when connecting the two circuits above. I thought I had the issue resolved. In order to start from scratch again, I connected two lm35 to a solid brass mass side by side to ensure that the reference design (figure 14) matched any connection method that was required as per the application notes. I used figure 14 with a 6-inch cable with  separate cores (no screen) cable length for one of the lm35 units and the other was fed from the same 4.07 volt dc lithium battery supply but with a 1 meter cable (3 core screened cable) with the screen grounded to negative as per figure 12. 

When I measure the output figure 12 gives a value of 147mv and figure 14 gives 137 mv. If I disconnect the cable screen of figure 12 both provide the same reading138mv versus 139 mv within the limit of tolerance.

My question is why does figure 12 increase the voltage. The capacitance is 0.20nf between screen and negative for the 1 meter cable. Which circuit is the correct configuration?. I will try and put some images up also. Thanks

photos.app.goo.gl/kPiDjci23HfiAEPP6

  • 0
    TI__Mastermind 43755 points

    Hi Seamus,

    Section 8.1.1 Capacitive Drive Capability of the datasheet states that LM35 can only handle 50pF of output capacitance. Your cable far exceeds this. Disconnecting the shield removes some of this capacitance. The datasheet recommends Figure 13 and bypass capacitance for high capacitive loads. 

    thanks,

    ren

  • 0 in reply to Ren Schackmann
    Prodigy 20 points

    I connected up Fig 13 also and I had no luck also (reading 150mv for figure 13 and 140mv figure 14). I am wondering is the problem something to do with the fact that I have both sensors bolted to the same metal mass and they share some common path to interfere with each other. I am also using the same PSU for both circuits and the negative of the dual scope is sharing a common path. If I disconnect the scope and use figure 14 set up for both I get the same readings when I re-connect on the same metal mass. ?.

  • 0 in reply to Seamus Sheehy
    TI__Mastermind 43755 points

    Hi Seamus,

    Did you connect the resistor and capacitor components at the LM35 device, before the cable? You may need to observe the output with a voltmeter, due to input impedance of the oscilloscope. Even with the suggested filter, the LM35 may not be able to provide enough current for the capacitive load.

    thanks,

    ren

  • 0 in reply to Ren Schackmann
    Prodigy 20 points

    OK, Should I run two separate cores and no screen cable -there must be a cable design that I can use to eliminate the issue and have you any sudgestions on this?. Is the capacitance between the output and negative that causes the problem or is it between the positive and negative or both?. What is your thinking on connecting the resistor and capacitor before the cable?. I will try a multimeter . I am using a fluke 196c scope and I just looked up the  manual they talk about capacitance between probes. 

  • 0 in reply to Seamus Sheehy
    TI__Mastermind 43755 points

    Seamus,

    You shouldn't need separate cores. Standard twisted pair should be sufficient.

    Any capacitance on the output pin is problematic.

    The resistor and capacitor shown in the datasheet figures won't provide any benefit unless it is connected to the LM35 before the cable.

    Bypass capacitance at the supply pins is recommended for all integrated circuits. It helps to stabilize the device's supply in the face of actions it may need to take to stabilize it's own outputs.

    Your battery may not meet the minimum supply requirements (4V) of LM35, especially as the battery discharges.

    thanks,

    ren

  • 0 in reply to Ren Schackmann
    Prodigy 20 points

    I only used the multimeter and the problem was solved. The dual scope had capacitance between the leads that I was not aware of. When I connect any circuit from fig 12 or fig13 now with the one meter length of cable I get the same reading . The existing capacitance between the negative and output is 140pf before connecting the circuit. Is it important to use the 75 ohm resistor or why is this value selected in figure 13. If I use 100 ohm will this cause a problem?. Is figure 13 more robust if the cable is lengthened. When I connect the bypass capacitor 0.01 micro farad I do not see any difference but I will add this to the design based on your recommendation. Thanks for all your help.

  • 0 in reply to Seamus Sheehy
    TI__Mastermind 43755 points

    Hi Seamus,

    The text says that Figure 13 has better immunity to EMI (relative to Figure 12) while also providing the needed protection against capacitive cabling. I would expect that 100ohm is acceptable; you'll have to determine if its suitable for your application.

    thanks,

    ren